N. R. "Woody" Woodward developed the Consolidated Design (CD) numbering system which can be found illustrated in his book, The Glass Insulator In America - 1988 Report. 

Q: Ron Barth asked if some informative articles on how some of the Hemingray or other manufacturing styles were used, such as CD 175 and CD 137.

A: CD 137 and CD 175 are both secondary cross-arm and drop line insulators. They were used before the development of the present-day coaxial drops that are in general use.CD137 is a compact style for #6 and #8 weatherproof insulated wire. Although they were a part of the "D" series of insulators introduced in the late 1920s, they never became common although they were available as D-990 for a number of years. They were at one time common in the Detroit area.

CD 175 (Hemingray-25) was introduced in the mid-1920s. They are found in blue-aqua and less often in ice green or blue-green. The only others I've seen are clear and were made in 1939. The earlier ones were used in great numbers by Public Service Co. of Northern Illinois (serving suburbs north of Chicago and now a part of Commonwealth Edison). The only place I've seen the 1939 production is in Bremerton, Washington, where they were used in a military housing project during World War II. After the war the project was taken over by the City of Bremerton Housing Authority. But don't catch the next flight for Seatac Airport because I believe that Marion Milholland got the last of them with my assistance about 25 years ago! 

As for the use of CD 175: in areas where there are houses or other buildings close together, more than one drop wire was attached to one pole. Rather than separate insulators, 2 to 4 drops were attached to the same insulator. I have seen cases where so many wires were tied to a CD 164 that the insulator was completely covered! The CD 175 was designed to hold the drop wires more efficiently; one wire tied above the other, three or four deep. 

When indicating the location where a particular insulator was used, one must always remember that is where the writer has seen them. In most cases, they may have been used elsewhere.